The present invention relates to a heavy duty pneumatic radial tire for running on rough ground surfaces, more particularly to a heavy duty pneumatic radial tire for running on rough ground surfaces which is capable of possessing both an envelope property and a cutting resistance of a shoulder portion, without degrading tire durability.
For example, a heavy duty pneumatic radial tire for use in large construction vehicles such as loaders receives shocks and cutting scratches on its tread portion by various projections such as broken stones, when it runs on rough ground surfaces. For this reason, cutting resistance which prevents the cutting scratches from easily reaching an inner surface of the tire, as well as an envelope property which follows so as to envelop the projections on the surfaces are required for structures of the tread portion.
Accordingly, heavy duty pneumatic radial tires for running on rough ground surfaces generally use non stretchable steel cords of a breaking elongation ratio of 4% or less for their reinforcement cords of belt layers in the tread portion, resulting in security of the cutting resistance property of the tire. Moreover, the whole width of the foregoing belt layers are made narrower than that of other belt layers used for general purpose radial tires in order to reduce the bending rigidity of the shoulder portion, whereby the foregoing heavy duty pneumatic radial tires are designed so as to possess the envelope property. However, although narrowing the whole width of the belt layers gives the envelope property for the foregoing radial tires, the cutting resistance at the shoulder portion is deteriorated.
For a countermeasure to the cutting resistance of the shoulder portion, as shown in FIG. 5, there has been proposed a technology that among a plurality of belt layers 17 arranged in a tread portion a width of the innermost belt layer 17X alone is extended to a shoulder portion 11A so as to widen the layer 17X, in a heavy duty pneumatic radial tire for running on rough ground surfaces which comprises a tread portion 11, a side wall portion 12, a carcass layer 14 and belt layers 17. However, the belt layer 17X of a high bending rigidity extends deeply to the shoulder portion 11A which is greatly deformed, whereby an edge thereof is easy to have an edge separation. Therefore, the belt layer 17X can not extend to the shoulder portion with a large width to keep good a belt durability. Accordingly the cutting resistance can not improve much. Additionally, as the belt layer of high bending rigidity extends to the shoulder portion, the envelope property is deteriorated.
Moreover, as another countermeasure other than the above, there is a technical proposal that a belt layer 17Y made of a stretchable steel cord of a breaking elongation ratio of 6% or more is provided on the outermost side of the belt layer 17 so as to extend to the shoulder portion 11A with a large width. However, the additional provision of the belt layer 17Y in the outside of the belt layer 17 increases the thickness of the tread portion, resulting in an increase in generation of heat. Therefore, this proposal necessarily solves the problem that the edge separation tends to occur in the edge portion so that the belt durability is reduced.